1. Field of the Invention
This invention relates to a control circuit for a motor and more particularly to a control circuit arranged to satisfactorily control the rotational phase of the rotor of the motor.
2. Description of the Prior Art
In controlling the rotational phase of the rotor of a motor, applying control solely to the rotational phase may be satisfactory for a motor where the rotor has a large moment of inertia, where the load on the rotor is small or has little fluctuation, or where a high degree of rotational precision is not required. However, where the moment of inertia of the rotor is small, where the load fluctuates to a great degree, or where a high degree of precision is required for the rotor, simple phase control is unstable and often cannot meet performance requirements. In such cases, therefore, speed control is generally provided in addition to phase control. In another known method, a negative feedback is arranged to decrease the current supply to the winding of the motor when the current supply increases.
The kinds of apparatuses incorporating servo motors have generally become smaller in size. Accordingly, the servo motor used in these kinds of apparatuses must also be smaller. Reduction in the size of the motor reduces the motor's torque. As a result of this smaller motor torque, when the normal load fluctuates due to changes in temperature, or in the kind of the load, etc., the voltage applied to the driving winding of the motor must be changed in accordance with the load torque to insure that the rotor maintains a constant rotational phase and speed. Since the speed detecting component produces a constant signal as the rotor rotates at a predetermined speed, the phase detection component must compensate for the degree of change (.DELTA.v) in the voltage applied to the driving winding due to the fluctuation in load torque. However, if the gain of the phase control loop is for example. A, the phase would deviate from a predetermined desired phase by as much as .DELTA.v/A.